New Ways To Incorporate
ARS plant geneticist Bob Hanneman and
University of Wisconsin graduate student Miguel Ramon in Madison, Wisconsin,
have developed new ways to incorporate the natural resistance in wild Mexican
species into commercial potatoes. They have crossed a sexual hybridization
bridge: Mating the wild Mexican speciesS. pinnatisectumwith
a derivative of a commercial potato variety, using a technique known as embryo
rescue. A hybrid from the rescue could be used as a maternal parent in a mating
with the cultivated potato.
"The group of Mexican species we are interested in have two sets of
chromosomes and are a rich genetic resource with extreme resistance to viruses,
insects, fungi, and nematodes," says Hanneman. "But they are
difficult to cross with the majority of other cultivated or wild species."
S. pinnatisectum is maintained at the U.S. Potato Genebank in
Sturgeon Bay, Wisconsin. ARS potato geneticist John Bamberg manages the
genebank, which receives and maintains wild species from plant collectors like
ARS botanist and plant explorer David Spooner in Madison.
"The problem of incorporating useful genes from any wild species is
the various barriers to crossing with the cultivated potato so that fertile
hybrids can be obtained. Those hybrids must be used in a breeding scheme that
eliminates undesirable traits contributed by the wild species," says
"To date, 16 wild species have been incorporated into commercial
varieties, but many more species have potential for use in breeding," says
Hanneman's early research crossed non-tuber-bearing wild potato species with
other wild species to find a way to access their germplasm by sexual means.
"S. verrucosum stood out as a potential parent because of the good
embryo development of the hybrids," he says.
So Hanneman selected S. verrucosum to cross with the
"uncrossable" Mexican potatoes. This mating yielded several new
hybrids, which have since been crossed with a range of wild relatives and with
derivatives of commercial varieties.
Lots of Good Traits To Choose From
The wild Mexican species also resist early blight, which is associated with
dry weather. Typically, the disease is seen in August when plants start to
mature. The fungus Alternaria solanithe culprit in early
blightcauses problems similar to those of late blight. But late blight
attacks quickly and is capable of defoliating a field within a matter of weeks.
Early blight is slower and progressive. Resistance to both diseases is needed
to reduce reliance on chemicals.
University of Wisconsin graduate student Joe Kuhl, working with Hanneman and
ARS plant geneticist Michael Havey, has advanced the research by identifying
the chromosome in S. pinnatisectum that confers resistance.
"We know there are a number of resistance genes in cultivated potatoes,
but the fungus has matching genes that negate the action of the host's
resistance genes. That means eventually the fungus may overcome this type of
resistance. Ideally, we'd like to find combinations of multiple resistance
genes, which could be more durable and not easily overcome by the fungus,"
The ultimate resistance test is always in the field. Last summer, the
researchers challenged the pinnatisectum hybrid against late blight at
the University of Wisconsin's Agricultural Research Station in Hancock. It
showed nearly 100 percent resistance to late blight. In replicated trials, the
hybrid was also resistant to Colorado potato beetle, an insect costing U.S.
potato, tomato, and eggplant growers about $150 million annually.By
Linda McGraw, Agricultural
Research Service Information Staff.
This research is part of Plant, Microbial, and Insect Genetic Resources,
Genomics, and Genetic Improvement, an ARS National Program (#301) described on
the World Wide Web at http://www.nps.ars.usda.gov/programs/cppvs.htm.
To reach the scientists mentioned in this article, contact
Linda McGraw, USDA-ARS
Information Staff, 1815 N. University St., Peoria, IL 61604; phone (309)
681-6530, fax (309) 681-6690.